The present invention relates to planar flexible graphite articles, such as flexible graphite sheet, and to a system and method for continuously producing such articles. More particularly, the present invention relates to flexible graphite sheet material that exhibits enhanced isotropy with respect to thermal and electrical conductivity and fluid diffusion, as well as to a method for producing the sheet.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional, e.g., thermal and electrical conductivity and fluid diffusion. Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the xe2x80x9ccxe2x80x9d axis or direction and the xe2x80x9caxe2x80x9d axes or directions. For simplicity, the xe2x80x9ccxe2x80x9d axis or direction may be considered as the direction perpendicular to the carbon layers. The xe2x80x9caxe2x80x9d axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the xe2x80x9ccxe2x80x9d direction. The natural graphites suitable for manufacturing flexible graphite possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Graphites, especially natural graphites, can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the xe2x80x9ccxe2x80x9d direction and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Natural graphite flake which has been expanded and more particularly expanded so as to have a final thickness or xe2x80x9ccxe2x80x9d direction dimension which is at least about 80 or more times the original xe2x80x9ccxe2x80x9d direction dimension can be formed without the use of a binder into cohesive or integrated sheets, e.g., webs, papers, strips, tapes, or the like. The formation of graphite particles which have been expanded to have a final thickness or xe2x80x9ccxe2x80x9d dimension which is at least 80 times the original xe2x80x9ccxe2x80x9d direction dimension into integrated sheets by compression, without the use of any binding material is possible. It is believed that this is due to the excellent mechanical interlocking, or cohesion that is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, such as roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material comprises compressing or compacting under a predetermined load and preferably in the absence of a binder, expanded graphite particles which have a xe2x80x9ccxe2x80x9d direction dimension which is at least 80 times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles are generally worm-like or vermiform in appearance, and once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 5 pounds per cubic foot to about 125 pounds per cubic foot. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the xe2x80x9ccxe2x80x9d direction and the directions ranging along the length and width, i.e., along or parallel to the opposed, major surfaces comprises the xe2x80x9caxe2x80x9d directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the xe2x80x9ccxe2x80x9d and xe2x80x9caxe2x80x9d directions.
This very considerable difference in properties, i.e., anisotropy, which is directionally dependent, can be disadvantageous in some applications. For example, in gasket applications where flexible graphite sheet is used as the gasket material and in use is held tightly between metal surfaces, the diffusion of fluid like gases or liquids occurs more readily parallel to and between the major surfaces of the flexible graphite sheet. It would, in most instances, provide for greater gasket performance, if the resistance to fluid flow parallel to the major surfaces of the graphite sheet (xe2x80x9caxe2x80x9d direction) were increased, even at the expense of reduced resistance to fluid diffusion flow transverse to the major faces of the graphite sheet (xe2x80x9ccxe2x80x9d direction). With respect to electrical properties, the resistivity of anisotropic flexible graphite sheet is high in the direction transverse to the major surfaces (xe2x80x9ccxe2x80x9d direction) of the flexible graphite sheet, and very substantially less in the direction parallel to and between the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction). In applications such as seals or other components (such as fluid flow field plates or gas diffusion layers) of fuel cells, it would be of advantage if the electrical resistance transverse to the major surfaces of the flexible graphite sheet (xe2x80x9ccxe2x80x9d direction) were decreased, even at the expense of an increase in electrical resistivity in the direction parallel to the major faces of the flexible graphite sheet (xe2x80x9caxe2x80x9d direction).
With respect to thermal properties, the thermal conductivity of a flexible graphite sheet in a direction parallel to the upper and lower surfaces of the flexible graphite sheet is relatively high, while it is relatively very low in the xe2x80x9ccxe2x80x9d direction transverse to the upper and lower surfaces. At times, and in certain applications, such as thermal interfaces, it may be desirable to increase the thermal conductivity of the sheet in the xe2x80x9ccxe2x80x9d direction.
In some applications, it is important to incorporate additives in the flexible graphite sheet in order to achieve corrosion resistance and to impregnate the flexible graphite sheet with resins and/or other material to increase the strength and water resistance of the flexible graphite sheet. Also, it is important at times to provide such additives in the course of processing the natural graphite into flexible graphite.
These foregoing situations are accommodated by the present invention.
In accordance with the present invention, a flexible graphite article in the form of a sheet having opposed, relatively planar, major surfaces is provided. The article is formed of particles of expanded (or exfoliated) graphite, an optically detectable portion of which, at magnifications of 100xc3x97 or less, are substantially unaligned with the opposed planar major surfaces of the flexible graphite article. Preferably, at least a portion of the unaligned particles are transverse to the opposed major surfaces of the article. The flexible graphite article is characterized by having decreased electrical resistivity and increased thermal conductivity in a direction transverse to the opposed planar major surfaces of the flexible graphite sheet and increased resistance to fluid flow in a direction parallel to the opposed planar major faces of the flexible graphite sheet. The flexible graphite sheet, with or without additives and/or impregnants, can be mechanically altered, such as by embossing, die molding and cutting to form components for electrochemical fuel cells, gaskets and heat conducting and heat resistant articles.
The present invention also includes an apparatus, system and method for producing flexible graphite sheet articles, such as those having decreased electrical resistivity and increased thermal conductivity in a direction transverse to the opposed planar major surfaces of the flexible graphite sheet and increased resistance to fluid flow in a direction parallel to the opposed planar major faces of the flexible graphite sheet.
The inventive method comprises reacting raw graphite particles with a liquid intercalant solution to form intercalated graphite particles; exposing the intercalated graphite particles to a temperature of at least about 700xc2x0 C. to expand the intercalated graphite particles to form a stream of exfoliated graphite particles; continuously compressing the stream of exfoliated graphite particles into a continuous coherent self-supporting mat of flexible graphite; continuously contacting the flexible graphite mat with liquid resin and impregnating the mat with liquid resin; and continuously calendering the flexible graphite mat to increase the density thereof to form a continuous flexible graphite sheet having a density of from about 5 to about 125 lbs/ft3 and a thickness of from about 1.0 to 0.003 inches.
The method also advantageously includes mechanically deforming a surface of the continuous flexible graphite sheet to provide a series of repeating patterns on a surface of the flexible graphite sheet or the removal of material from the flexible graphite sheet in a series of repeating patterns and vaporizing at least some of the solvent from the resin prior to mechanically deforming a surface of the continuous flexible graphite sheet.
As noted, the present invention also includes an apparatus for the continuous production of resin-impregnated flexible graphite sheet, comprising a reactor vessel for containing as reactants graphite particles in mixture with a liquid intercalant solution to form intercalated graphite particles; an expansion chamber in operative connection with the reactor vessel, the interior of the expansion chamber being at a temperature of at least about 700xc2x0 C. (and preferably enclosing an open flame), such that passing intercalated graphite particles from the reactor vessel to the expansion chamber causes expansion of the intercalated graphite particles to form exfoliated graphite particles; a compression station positioned to receive exfoliated graphite particles for compressing such particles into a coherent self-supporting mat of flexible graphite; an impregnation chamber for contacting the flexible graphite mat with liquid resin and impregnating the mat with the liquid resin; a calender mill disposed to receive the flexible graphite mat for increasing the density of the mat to form a continuous flexible graphite sheet preferably having a density of from about 5 to about 125 lbs/ft3 and a thickness of no more than about 1.0 inches, more preferably about 1.0 to about 0.003 inches.
The inventive apparatus also preferably includes a device for mechanically deforming a surface of the continuous flexible graphite sheet to provide a series of repeating patterns on a surface of the flexible graphite sheet or the removal of material from the flexible graphite sheet in a series of repeating patterns. It further advantageously has an oven for receiving the mat from the device for mechanically deforming a surface of the continuous flexible graphite sheet, to cure the resin with which the continuous flexible graphite sheet is impregnated.
In a particular embodiment of the invention, a system for the continuous production of surface patterned, resin-impregnated flexible graphite sheet is presented. The system includes:
(i) a reactor vessel for containing as reactants raw natural graphite flake-like particles in mixture with sulfuric and nitric acids;
(ii) an acid containing vessel communicating with said reactor vessel for the introduction of a mixture of sulfuric and nitric acid into said reactor vessel;
(iii) a graphite particle containing vessel for the introduction of graphite particles into the reactor vessel;
(iv) a first additive containing vessel communicating with said reactor vessel for the introduction of intercalation enhancing materials, acids or organic chemicals;
(v) a wash vessel containing water communicating with the reactor vessel to receive reaction product in the form of acid intercalated graphite particles and remove acid from the surface of the acid intercalated graphite particles and a portion of the mineral impurities contained in the natural graphite particles introduced into the reactor vessel;
(vi) a drying chamber for drying washed acid intercalated graphite particles;
(vii) conduit means extending from said wash vessel to said drying chamber for passing washed acid intercalated graphite particles from the wash vessel to the drying chamber;
(viii) a second additive containing vessel communicating with the conduit means of (vii) for adding pollution reducing chemicals to the washed, intercalated graphite particles to the washed acid intercalated graphite particles;
(ix) a collecting vessel for collecting washed acid intercalated graphite particles admixed with pollution reducing chemicals;
(x) conduit means extending from said drying chamber to said collecting vessel for passing acid intercalated graphite particles admixed with acid additives from said drying chamber to said collecting vessel;
(xi) a third additive containing vessel communicating with said conduit of (x) for the introduction of ceramic fiber particles in the form of macerated quartz glass fibers, carbon and graphite fibers, zirconia, boron nitride, silicon carbide and magnesia fibers, naturally occurring mineral fibers such as calcium metasilicate fibers, calcium aluminum silicate fibers, aluminum oxide fibers and the like into said conduit and the admixing and entrainment thereof with acid intercalated graphite particles passing from the washing vessel to the drying chamber;
(xii) an expansion chamber enclosing an open flame at a temperature of 800 to 1300xc2x0 C.;
(xiii) conduit means extending from said collecting vessel to said expansion chamber for passing dried acid intercalated graphite particles admixed with ceramic particles to said expansion chamber;
(xiv) gas inlet means communicating with the conduit means of (xii) for entraining the acid intercalated graphite particles admixed with ceramic particles in a stream of non-reactive gas and passing the entrained acid intercalated graphite particles admixed with ceramic particles into the open flame enclosed in said expansion chamber to cause expansion of the acid intercalated graphite particles of at least about 80 times to form vermiform elongated graphite particles;
(xv) a collecting hopper for receiving said vermiform elongated graphite particles admixed with ceramic particles;
(xvi) a separator vessel interposed between the expansion chamber and the collecting hopper to collect by gravity separation heavy solid mineral impurity particles from the mixture of vermiform graphite particles with ceramic particles;
(xvii) a gas scrubber communicating with said collecting hopper to collect gases generated in the expansion chamber;
(xviii) a compression chamber positioned to receive vermiform graphite particles mixed with ceramic fiber particles for compressing said vermiform particles mixed with ceramic particles into a coherent self-supporting mat of flexible graphite from about 1 to about 0.015 inches in thickness and having a density of from about 5 to about 25 lbs./ft.3;
(xix) an impregnation chamber for contacting the flexible graphite mat of (xviii) with liquid resin and impregnating said flexible graphite with liquid resin;
(xx) a dryer disposed to receive the impregnated flexible graphite mat of (xix) and heat and dry said mat;
(xxi) a calender mill disposed to receive the flexible graphite mat of (xix) for increasing the density of said flexible graphite mat to form a continuous flexible graphite sheet having a density of from about 5 to about 80 lbs/ft3, a thickness of from about 0.5 to about 0.005 inches and relatively evenly spaced apart opposite surfaces;
(xxii) a device for mechanically deforming a surface of the continuous flexible graphite sheet of (xxi) to provide a series of repeating patterns on said surface flexible graphite sheet or the removal of material from said flexible graphite sheet in a series of repeating patterns; and
(xxiii) an oven for receiving the mat from the dryer of (xxii) to cure the resin the mat.